Remus Lupin

Detention was supposed to be James Potter's punishment. At least, that's what Remus Lupin thought.
But after another of James's pranks, Professor Slughorn somehow decided that both were to blame and sent Remus and you to clear out the old pantry of ingredients.
Before leaving, James only smiled contentedly at Remus:
— Believe me, Moony. I've thought of everything.
Remus didn't understand then what exactly he had thought of.
While Slughorn was explaining the task, James quietly disappeared after the professor left. A few minutes later, the storeroom door closed behind you.
Remus looked at her for a few seconds.

• He did it on purpose...
  He ran a hand through his hair and sighed softly. Of course. James knew perfectly well that Remus hadn't been able to talk to you properly for months without blushing.
  You were one of the smartest students in your year—quiet, well-read, and attentive to detail. You were often teased for your quietness, especially by those who assumed that someone who rarely spoke simply couldn't hold a conversation. But Remus knew that wasn't true at all.
  He loved the way you could spend hours reading books, memorizing details, and then suddenly making a perfectly accurate remark. That was probably what he noticed about you first. In a way, you reminded him of himself.
  And, what was especially painful, you were no less embarrassed of him than he was of you.
  "Never mind," Remus muttered, clearly embarrassed. "Potter's just... an idiot."
  The pantry was silent. The high shelves were filled with dozens of vials and jars of ingredients.
  While Remus was busy with his assigned task, you began examining the unusual potions. A potion of sudden attraction, a beetroot juice stain solution, and so on.
  "Just don't touch anything," he warned softly, noticing your interest. "It's better not to take any risks."
• I'll just have a look.
  — I know. But that's usually how it all starts.
  You chuckled and reached for a small bottle on the top shelf, but still managed to drop it.

She broke the potion "Sudden Attraction", which temporarily affects two people, making their existing romantic feelings much stronger.